The invention is in the field of hydraulic valve lifters, and more particularly pertains to variable duration valve lifters.
Hydraulic valve lifters are a blessing because they adjust the valve train, if necessary, every time the valve seats. Variable duration hydraulic valve lifters are an improvement upon the basic hydraulic lifter which has application in high-performance engines. The radical cam of the high performance engine causes valve overlap, which is effective at improving engine performance at high speeds, but causes very rough engine performance at idling. The variable duration, or "bleed down", lifter solves this problem. The instant inventor has two prior patents on bleed-down lifters, the first of which is U.S. Pat. No. 3,921,609, issued on Nov. 25, 1975. The second is a modification and improvement on the first, and bears U.S. Pat. No. 4,524,731, issued June 25, 1985. Thorough descriptions of the purpose and functioning of bleed-down hydraulic lifters are found in those patents. The instant invention applies to these lifters as well as other types of lifters, such as the so-called "sloppy fit" lifter in which bleed-down occurs between the plunger and the outer cylinder.
Unfortunately, variable duration, or bleed-down lifters cause the engine to tick rather loudly at idling speeds. The ticking comes from the closing of the valves in their seats at a higher speed than that for which quiet operation is maintained. The accelerated seating speed in turn is caused by two separate factors.
First, bleed-down lifters cause the valve train to continuously shorten from the time the lifting cycle begins until the valve seats again. The shortening of the effective valve train while the valve is seating accelerates the impact of the valve in its seat.
Secondly, because the variable duration lifter moves forward in time the point at which the valve seats, the valve will seat at a higher point on the closing ramp on the cam. The higher point on the closing ramp is steeper and is thus bringing the valve into the seating position at a higher velocity than would be the case if the valve were to seat later as it normally would.
This invention is principally targeted toward these two problems, with the aim of reducing or eliminating valve ticking.